The present invention relates generally to a digital X-ray imaging system and more specifically to a method of sensing temperature of a digital X-ray imaging system.
X-ray imaging systems, also known as X-ray detectors, have become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. One category of X-ray imaging systems uses scintillator materials located on an array of photodiodes and FET""s to convert X-ray photons into visible-spectrum photons as part of the energy detection process. The photodiodes and FET""s are located on a glass substrate panel. Since charge leakage from the diodes is an exponential function of temperature, the pixel outputs of the photodiodes and FET""s are strongly dependent upon the temperature of the glass substrate panel.
For this reason, it is necessary to maintain the detector panel temperature within a narrow operating range, and to correct for images taken with the X-ray imaging system with an xe2x80x9coffset imagexe2x80x9d taken without X-ray. The term xe2x80x9coffset imagexe2x80x9d is used here to refer to an image that is taken from the X-ray imaging system without X-ray illumination, and which represents the output of the detector due to confounding factors including among other things diode leakage, charge retention, and electronic noise. Differences in digitized output values for a pixel""s diode in an offset image that correlate differences in temperature of that pixel""s diode are considered to be primarily due to diode leakage. Diode leakage is also known as dark current because it is the current the diode is passing while not illuminated.
Known detectors are cooled with liquid coolant flowing in a coldplate in the detector, with heat removed by a remotely mounted chiller. Temperature sensing is done with temperature sensors located on a circuit board in the detector near the glass substrate panel.
However, this type of temperature sensing has inherent errors. For example, the sensors are not in physical contact with the X-ray detector panel. Further, the number of sensors is limited both by cost and space available. Also, there are heat-dissipating components on the circuit boards which affect the temperature sensors. Thus, the temperature on the panel and the spatial distribution of temperature across the panel are known only approximately.
It is therefore highly desirable to provide a direct measure of the panel temperature and a better representation of the spatial distribution of temperature across the panel. Direct measurement of the panel temperature will enable improved closed-loop control of the detector cooling system. Knowledge of spatial distribution across the panel from direct measurement will enable the use of other cooling methods without the risk of some areas of the panel being outside the required temperature range for imaging.
The present invention uses the leakage (dark current) of the X-ray detector panel""s diodes to provide more accurate data about the temperature of the X-ray detector panel.
To accomplish this, offset images are taken at known temperatures when the X-ray panel is manufactured. Offset values are recorded for each diode (pixel) at two or more known temperatures. A temperature versus offset curve is the created for each pixel. When the detector is installed into an imaging system, this data is loaded into the system for use by the imaging acquisition software. Upon subsequent use of the X-ray imaging system, values from the offset images, taken without X-ray either immediately before or immediately after the X-ray image are taken, are used with the temperature dependent coefficients of some or all of the diodes on the panel to calculate the temperature of the panel at the time of the offset image acquisition. The temperature of the panel and spatial distribution of temperature across the panel determined in this way may then used to regulate the cooling system of the detector to maintain the panel within the temperature range required for imaging.
This method will also allow for the use of cooling methods that are not presently available to known systems such as direct conduction cooling with heat pumped by thermoelectric coolers. This method also eliminates the need for thermal sensors on the circuit boards of the X-ray imaging system, which saves costs in terms of manufacturing and reliability.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.